Items 243-246

243. Microfilaments

(A) are 4-7 nm in diameter
(B) contain the proteins actin and keratin
(C) cause the anterograde movement of mitochondria in axons
(D) form the dense mass of filaments attaching to desmosomes
(E) bind chromosomes to the mitotic spindle

244. Actin

(A) is a globular protein that can into filaments
(B) makes up most of the M-line in skeletal muscle
(C) causes the movements of vesicles along the microtubules of axons
(D) contains specific binding sites for the enzyme dynein
(E) can be degreded, by certain proteases, into heavy and light meromyosins

245. The actin in non-muscle cells

(A) is usually associated with dynein
(B) forms filaments that are frequently assembled and disassembled
(C) is usually associated with non-muscle tropomyosin
(D) rarely makes contact with the plasma membrane]
(E) is usually found in sarcomeres

246. If antibody that is specific for myosin is microinjected into a cell that has just completed DNA synthesis, which of the following is the most likely consequence?

(A) Cell division will be completed normally, but the daughter cells be unable to move during the following interphase.
(B) Cytokinesis will proceed normally, but chromosome movement at the ensung anaphase will be prevented.
(C) Injected cells will enter prophase but not proceed to metaphase because no mitotic spindle will assemble.
(D) The cell will fail to divide, but will subsequently be found to have two nuclei.
(E) There will be no discernible effect because non-muscle cells contain actin but no myosin.

ANSWERS AND TUTORIAL ON ITEMS 243-246

The answers are: 234-A; 244-A; 245-B; 246-D. The protein actin is found in essentially all eukaryotic cells. In non-muscle cells actin forms filaments with a diameter of 4-7 nm, called microfilaments. In contrast to the thin filaments in muscle cells, which are fairly stable assemblies of actin, the microfilaments of non-muscle cells are dynamic structures that are frequently assembled and disassembled in response to the multiple stimuli to which non-muscle cells must respond. Most non-muscle cells also contain one or more types of myosin, with similarities to and differences from the myosins found in muscle cells.

Actin microfilaments participate in a variety of cellular activities, many of which require the participation of non-muscle myosins. One such activity is cytokinesis, the process by which a contractile ring – containing myosin and actin microfilaments – causes a constriction at the midbody of the mitotic spindle. This constriction causes the daughter nuclei to be separated from one another into distinct cytoplasmic domains – the daughter cells. If myosin activity is disrupted by microinjection of specific antibodies the various stages of mitosis can proceed “normally” but the contractile ring fails to form and/or function properly; the result is the formation of cells with the daughter nuclei contained in a single common cytoplasm. USB Computer Connected Microscopes Video Eyepiece Cameras Video Zoom Inspection microscopes